The invention relates to a method of welding parts together by means of laser beams.
Such methods are known, butxe2x80x94although performing well in many applicationsxe2x80x94present a number of shortcomings. One problem is that when using such methods, parts are contaminated due to evaporated matter which re-deposits on the parts during the welding process. Another problem is that such methods require precise tolerances for the gap between the parts during the welding process.
In the method known from EP687519, parts to be welded are pressed together in the vicinity of the laser beam in order to ensure that the maximum gap between parts is not exceeded. This method considerably constrains handling of the parts prior to and during the welding process, as well as the parts themselves. Moreover, it does not solve the problem of contamination.
Product contamination and/or bad welds due to incorrect distance between parts may lead to the product being rejected, thereby reducing the efficiency of the process.
It is an object of the invention to provide a method of laser welding having an improved efficiency.
To this end, the method is characterized in that it proceeds in at least two steps comprising:
a first step wherein at least one melt is created in at least one part at a first laser intensity, and
a further step wherein said at least one melt is pushed towards a further part at a second laser intensity, higher than said first laser intensity, whereby said at least one part and said further part are welded together.
Such a method is more efficient because a smaller number of products will be rejected due to contamination and/or bad welds.
Contamination is mainly due to matter which is vaporized during the welding process and re-deposits on the product. There is almost no contamination in the first step of the method because it proceeds essentially with melting the matter without or with very little evaporation. In the further step of the method, a higher laser intensity is used in order to evaporate some matter, thereby creating a vapor recoil pressure which will push the melt created during the first step towards the further part. Due to the molten state of said melt, the amount of energy required to push said melt is reduced and therefore the amount of evaporated matter is reduced too. Contamination is consequently reduced in the further step.
Bad welds occur when the gap between parts is too large to be bridged by the melt. In the further step of the method, the melt of one part is pushed towards a further part, thereby allowing to bridge large gaps between these parts. The method thus increases the tolerated gap between parts to be welded and consequently reduces the risk of bad welds.
The method is preferably applied to spot welding of parts. Within the concept of the invention, the gap between parts to be welded (gp) corresponds to the distance between facing sides of said parts, taken along the axis of the laser beam. The maximum which can be bridged by the weld depends on the thickness (ep) of the part in which the melt is first created, said thickness being the distance taken along the laser beam axis between opposite sides of said part.
When applying the method according to the invention, clean and proper spot welds are achieved, even when gpxe2x89xa70.4ep, a result which is difficult to achievexe2x80x94if at all achievablexe2x80x94with prior art laser welding methods.
In particular embodiments, clean and proper spot welds are achieved even when gpxe2x89xa71.0ep, which represents a gain in tolerated gap by a factor of two and a half compared to prior-art laser welding methods, yet reducing contamination e.g. by a factor of two.
After said further step of the method, it may be advantageous to proceed with a subsequent step comprising a post-heating of the weld at another laser intensity in order to increase the melt volume, again without or with very little evaporation of matter.
Furthermore, various positions of the laser beam with regard to the parts to be welded are possible. In preferred embodiments, the laser beam comes from above the part in which the melt is created, In other embodiments, the laser beam may also come from below or from beside the part in which the melt is created.
The laser beam preferably strikes the part in which the melt is to be made substantially at right angles, but, in embodiments in which the parts to be welded are difficult to reach, the laser beam may strike under an angle.
In particular embodiments, more than one laser beam may be used. In a preferred case, two laser beams are used, a first one coming from above a first part, a second one coming from below a further part, thereby allowing to bridge even larger gaps up to the sum of the thicknesses of the parts between parts than with a single laser beam.
The method is preferably applied to laser welding of metallic parts.
The second part may be formed by or comprise a metallic layer on a substrate. The substrate may be a ceramic substrate or a flexible substrate (such as a plastic film) or, for instance, a printed circuit board.
The method is of particular use in embodiments in which the second part is vulnerable to heat input or to contamination. This may be the case if a metal part has to be connected to or near an electronic device (for instance, a printed circuit board). In embodiments where one laser beam hitting the first part is used, said laser beam does not penetrate through the first part, or at least the chance of this happening is very small, so that direct laser heat input in the second part is small. Furthermore, contamination is reduced.
The method in accordance with the invention also enables materials to be welded which can otherwise not be welded, or only with great difficulty. One such example is a weld of stainless steel (as a first part) on aluminium (as a second part or the top layer of the second part). It has been found that such a weld can be made easily and without great contamination by means of the method in accordance with the invention, whereas in conventional laser welding methods, this is difficult, if not almost impossible, at least not without using excessive laser powers and the resulting extensive contamination.